HAVANA, July 7 (Reuters) – Cuba struggled on Tuesday to restore power after its national grid collapsed the day before, leaving millions still without electricity across the Caribbean island nation.

Authorities have yet to fully explain the cause of the Monday blackout—the third to affect the country this year—which left an already exhausted population of nearly 10 million people in the dark overnight.

The country’s grid operator UNE said early on Tuesday it had reconnected central Cuba to the grid, from the western port of Mariel to Sancti Spiritus province, but that the eastern and western extremes of the island remained offline.

About one-third of the capital Havana had seen power restored on Tuesday morning, though large swaths were still without electricity and power was spotty even where it had been restored.

The latest grid collapse comes as the United States has vastly ratcheted up pressure on Cuba, cutting off the island’s fuel supply and applying severe sanctions in a bid by the Trump administration to force a change in Cuba’s communist-run government.

Cuba and the United Nations have called the Trump administration measures illegal and a violation of the human rights of the island’s residents.